1. Field of the Invention
This invention relates to crosslinked ethylene-olefin copolymer foams foamed from polyethylene-based resins polymerized by using a metallocene catalyst and a method for the production thereof.
2. Description of the Prior Art
Heretofore, crosslinked polyethylene foams have been produced by using low-density polyethylene or ethylene-vinyl acetate copolymer as a raw material and adopting the following process:
(a) one step foaming process in which a foamabale and crosslinkable composition charged into a mold in a press is heated therein under increased pressure and thereafter the pressure applied to the composition in the mold is released to give rise to a foam, or PA1 (b) two step foaming process, as disclosed in Japanese Patent Publication No. 45-29381 and No. 2-42649, comprising the first step in which a foamable and crosslinkable composition is heated in a closed mold under increased pressure to partially decompose a blowing agent and a crosslinking agent and the pressure is released to form an intermediate foam, and the second step in which the resultant intermediate foam is heated under atmospheric pressure to decompose the remaining blowing agent and crosslinking agent.
The foamed product obtained by the aforementioned one step foaming process is liable to deform during the course of expansion because the used polymer expands in a moment upon releasing the pressure. By the one step foaming process, therefore, there is barely obtained a foamed product of an expansion ratio up to about 15 times the original volume. On the other hand, a highly expanded foam is obtained by the two step foaming process. This process, however, has a disadvantage that the production cycle is longer than that of the one step foaming, which results in the higher manufacturing costs.
Although a low-density polyethylene foam obtained by any process exhibits excellent resistance to heat, it is lacking in flexibility and extensibility. An ethylene-vinyl acetate copolymer foam is excellent in flexibility and extensibility, but poor in heat resistance. So it can only be used at a temperature of 80.degree. C. at the most. Besides, since the conventional resins have a wide molecular-weight distribution, it is not easy to manufacture a crosslinked foam with uniform quality.
Recently there has been proposed a foam molding technique which uses a polyethylene-based resin polymerized by using a metallocene catalyst. For example, published Japanese Patent Application, KOKAI (Early Publication) No. (hereinafter referred to briefly as "JP-A-") 7-188442 proposes a method for producing a crosslinked, foamed sheet, which method comprises irradiating a sheet of such a polyethylene-based resin 1 mm in thickness with electron rays to effect crosslinking of the resin and then heating the sheet in an oven to effect expansion thereof, as disclosed in examples. JP-A-7-207054 proposes a method of foaming a thin sheet of a mixture of polypropylene and a polyethylene-based resin polymerized by using a metallocene compound in the same manner as the above method. A foaming method disclosed in JP-A-7-179640 comprises blending a polyethylene-based resin polymerized by means of a metallocene compound with (a) a halogen-containing flame-retardant and antimony trioxide or (b) red phosphorus and a nitrogen compound, forming a sheet of the resultant blend, irradiating the sheet with electron rays to effect crosslinking of the resin, and heating the sheet to produce a thin crosslinked, foamed sheet.
As mentioned above, various methods for producing foamed thin sheets by the electron radiation crosslinking have been proposed, but those for producing chemically crosslinked block foams have not been proposed yet.